Ultra high frequency electronic device



Oct. 1, 1946.

A. E. BOWEN 2,408g409 ULTRA HIGH-FREQUENCY ELECTRONIC DEVICE Filed April8, 1941 4 Sheets-sheaf 1 IN I/E/V TOR VAEBOWE/V A T TORNEV l-SheecS-Sheet 3 war A. E. BOWEN ULTRA HIGH-FREQUENCY ELECTRONIC DEVICEFiled. April 8, 194;

FIG W Oct. 1, 1946.

iii iiii. a, 1% f INVENI'OR AE. BOWEN ATTORNEY Patented Oct. 1, 1946ULTRA HIGH FREQUENCY ELECTRONIC DEVICE Arnold E. Bowen, Red Bank, N. J.,assignor to Bell Telephone Labora tories, Incorporated, New

York, N. Y., a corporation of New York Application April 8, 1941, SerialNo. 387,432

14 Claims.

This invention relates to resonators and resonant cavities so shaped andconstructed as to be suitable for use with an electron stream to enablean electromagnetic field which may be set up in the resonator orresonant cavity to induce a variation in some characteristic property ofthe electron stream, or, in general, to permit an interchange of energybetween the resonator and the electron stream.

In particular, it relates to arrangements for facilitating interchangeof energy between an electron stream and an electromagnetic field in aresonant cavity and for coupling such a resonator to an extended Waveguide for energy transmission or reception. suitable form, as, forexample, a hollow conductive tube containing air, or a rod of dielectricmaterial, etc.

The resonators employed in various embodiments of the invention are of anumber of types.

For example, the resonant cavity may consist of the space between twoconcentric cylindrical conductive shells of slightly different radii andslightly different lengths, each of the shells being closed at both endsby conductive discs except for such apertures or quired for couplingpurposes. resonator which is particularly well adapted for couplingbetween a Wave guide and an electron stream is formed by partitioningoiT a section of the Wave guide of proper length to serve as a resonatorat the desired operating frequency. Wave guides of either rectangular orcircular cross section are most conveniently used although Another formof the invention is not limited to any particular shape or size ofguide.

The methods of coupling the resonator to the wave guide that may be usedin accordance with the invention are various. A resonator may be coupledinto the side of a cylindrical wave guide through a connecting tube. Inthe case of a resonator partitioned ofi within an extended wave guidethe coupling may be by way of an aperture in the partition wall and theamount of the coupling may be made adjustable by means of an iris orother means of varying the size of the aperture.

Openings for the passage of the electron stream are generally providedat voltage anti-nodes of the electric field in order to secure themaximum interchange of energy. In one arrangement a plurality ofelectron streams are arranged in a ring concentric with the axis of aresonator, or a tubular-shaped electron stream may be employed.

The wave guide may be of any w perforations as may be re- The inventionis applicable generally to amplifiers, oscillators, modulators,detectors and the like, particularly at ultra-high frequencies, Whereverit is desired to effect direct interaction between an electromagneticfield and an electron stream.

The invention is described with reference to a number of illustrativeexamples.

In the drawings:

Fig. 1 is a perspective view, partly in cross section and partlydiagrammatic, showing an electron tube oscillator having a cavityresonator comprising concentric cylindrical shells;

Fig. 2 is a general perspective view of an oscillator of the type shownin Fig. 1, coupled to a long cylindrical wave guide;

Fig. 3 is a cross-sectional view partly in perspective with a detailedshowing of one arrangement for coupling the oscillator to the waveguide;

Figs. 4 and 5 show alternative forms of resonators comprising concentriccylindrical shells;

Fig. 6 shows another method of coupling an oscillator of the type shownin Fig. 1 to a wave guide and difiers somewhat in detail from thearrangement shown in Fig. 3;

Figs. 7 and 8 show variations of an oscillator of the type shown in Fig.1 modified so as to accommodate a plurality of electron streams;

Fig. 9 is an end view of an oscillator as in Fig. 7 or Fig. 8 showingthe arrangement of the vacuum tubes in a circular array;

Figs. 10 and 11 show a top view and a side view, respectively, partiallyin cross section, of a cavity resonator consisting of a compartmentpartitioned oil from the main portion of a wave guide, the compartmenthaving two semicircular walls one of which is slidably mounted fortuning purposes;

Figs. 12 and 13 show perspective views partially in cross section ofresonators employing sections of rectangular wave guides andaccommodating a linearly extended electron stream;

Fig. 14 is a detailed cross-sectional view of a cathode installationsuitable for use with the structures of Figs. 12 and 13;

Fig. 15 is a perspective view, partially cut away, of a form of diodeoscillator built into a rectangular wave guide;

Fig. 15A is a diagram useful in explaining the construction of andelectrical connections to the oscillator of Fig. 15;

Fig. 16 is a longitudinal cross-sectional view of a modification of thearrangement shown in Fig. 15; and

Fig. 17 shows an oscillator employing a wave will provide conductiveconnections between the I can l and cylinder 2 with negligibleobstruction to the passage of electromagnetic waves'throughout the spaceor cavity between thecylinders.

The latter may be made of copper or other suitable conductive material.Cylinder'i is provided with an axial shielded passageway or tube 4 andthe left-hand face of cylinder 5 is pierced by acoaxial hole A coppertube or other hollow conductive cylinder 5 is fastened tothe leit handrace of cylinder 1 and the tube E is joined by mean of a suitablehermetic seal"? to a glass or other suitableinsulating envelope 8..Within the envelope '8 are provided'the-elements of an electron gun ofany suitable type comprising, for example, a cathode 9, and anaccelerating electrode ill cooperating with the cylinder 6 'to direct anelectron beam through the hole 5 and the aiignedholes at the ends of thepassage 'or tube l. Gaps l i and-l5 are constituted as indicated in thepathof the beam. The gap i i is constituted between the-edges of thehole 5 and the left -hand-end of-tube is, and the gap is constitutedbetween theright-hand end of tube 5 and the inner surface of the can 4.Batteries H, l2 and i3 are provided respectively for heating'thecathodeii, energizing the accelerating electrode H3 and applyin'g anaccelerating potential "to the metallic' or conductive system comprisingthe cylinders i, 2 and although other suitable energizing means may besubstituted.

In the operation of'the system of Fig. l, oscillations are maintained inthe' resonant system comprising the space between cylinders i and 2 bymeans' of interaction between the electron stream andelectromagnetic'wavesin the resonator. Physically the action of. thedevice may be thought of as follows: I-n'their-passage across the gap il, the electrons either take from -'or give energy to thehigh frequencyelectromagnetic field in the'reso-nant cavity depending upon the phaseor" the field duringtheir transit across the gap. The velocities of theelectrons are varied inaccordance with the energy interchange inIWEl'l-kl'LOWIl manner. Then the electrons pass through the passagewayor tube i where a grouping or bunching eiiect takes place, thoseelectrons which. have lost energy and have as a consequence beenslowedup, being overtaken by other electrons 'WJIilCll have entered later;have gained energy and :t'husbeen speeded up. Consequently at, a pointsome distance to the right of the gap the electrons are traveling inmore or less well-defined groups. Upon reaching the ,secondgap iii the'lcunchesof electrons may, if the length of the passageway i; and theinitial speed of the electrons have beenadjusted correctly, cross thegapjfi in opposition; to the high irequencyj electromagnetic field; thuscontributing energy to the field, and in greater amounts than thatabsorbed by. he thinly distributed electronswhich may cross thegap i5during-the unfavorable phase of the high frequencyfield.

it; is usually d sirable thattheis'teady. com- 4 ponent of the potentialof the cylinder 2 be the same or nearly the same as that of cylinder i.To secure this condition, one face of the insulating or dielectric ring3 may be coated with a thin film of conducting material. Experimentswith such films in connection with wave guides have (indicated that a.fi'lmcan bernade to give sufficiently high conductivity for maintainingthe steady component of potential without impeding seriously the passageof the high frequency wave through the film. Alternatively the ring 3may be replaced if desired by one or more rods or studs of conductivematerial preferably spaced evenly about the periphery of the cylinder 2.

An oscillate-r ot the type shown in Fig. 1 may be coupled to a waveguide for transmission to a distant point. One coupling arrangement isillustrated generally in Fig. 2 wherein i5 is a circular wave guide toone side of whichis attached the cylinder 8. The details of the couplingbetween the oscillator and the wave guide may be arranged in a variety.of ways, oneoi which is shown in .Fig. 3.

The wave guide 58, illustrated as being, a, conductive "tube, is shownin cross section in 3. The resonant chamber. electron gun :arrange mentsand other details of the-oscillator zarexsimie lar to those --shown inFig. 1 with the principal exception of a change in the, right-hand wallof cylinder l. Anaxial hole i1 is cut through this wall and a metallicor conductive tube '18 is fastened overthe "hole. The end of the tube-l8is sealed ch, as, for example, by a metal glass seal is and a glass head.so as to complete-the closure-cf the vacuum-chamber. inside cylinder lis placed a disc 2| of conductive material parallel to the face ofcylinder I and separated from it by a small gap. A conductive rod 22' ispro-. vided which :serves'to support the disc 2,! and is in turn held invposition and sealedinthebead 21 In the system of Fig. 3oscillations'aremaintained in the resonant cavity in the mannerdescribed in connection with'Fig. 1 and in addition 'it is apparenththata portion oflthe high frequency energy resident in the resonant cavity 7will escape through the-gap between disc 2! and cylinder 1 and will beavailablerioruse .llJzOlltside circuits. In the arrangement illustratedin Fig. 3, the outside circuit is thewave guide i 16. Ifhe rod 22 isplaced across adiametercof the guide 46 and serves to establishaatransverse electric or H11 wave in the guide is ina wellknown manner.

The resonatorof Fig. l or Fig. 3 operates with voltage anti-nodes at thegaps I4 and iii, the in sulating ring 3. being locatedat a'voltage'nodein the equatorial plane. It is evident, therefore, that in designing theresonator for. .work'ingat a predetermined frequency, the'effectivelength of the cavity between the. cylinders-.1 anal from the gap. ill tothe gap i5 should ibe' made substantially .a half wavelength.Thewa-ve-leng th referred to here'is,iiof course; determined by thevelocity of propagation of the wave in the cavity.

' The best dimensions for a given frequency may inders of somewhatdifierent proportions from those shown in the preceding figures.

Fig. 6 shows an arrangement similar to that of Fig. 3 except thatprovision is made for slowing down the electrons before collection atthe anode. In the arrangements of Figs. 1 and 3, the electric fields inthe gap |5 will usually not be surficiently strong to produce the idealcondition in which many of the electrons would be brought almost to restand would strike the right-hand wall of cylinder in the arrangement ofFig. 1 or the disc 2| in the arrangement of Fig. 3 with very low kineticenergy. However, in practical arrangements of the type shown in Figs. 1and 3, the electron stream usually gives up only a small fraction of itsenergy to the field and the remaining energy is wasted in the form ofheat generated by the electrons striking the target. In the modificationshown in Fig. 6, provision is made for reducing losses of this kind. Inthe arrangement of Fig. 6 the disc 2| is pierced by an axial hole 23through which the electron stream may emerge from the resonant chamber.A collecting electrode 24 is maintained at a potential somewhat lowerthan that of the cylinders I and 2, this lower potential being suppliedby a battery 25. An axial hole is provided in the right-hand end ofcylinder l where a conductive tube 26 is attached. The disc 2| isattached to another conductive tube 27 coaxial with tube 2%. These twotubes are separated near the right-hand end by an insulating ring 28.The tube 27 extends to the right a little beyond the ring 28 where theend is hermetically sealed with a glass bead or other insulatingmaterial through which is also sealed a lead 29 which provideselectrical connection to and mechanical support for the anode 24. Thecylinder 26 is put through a hole in the wall of the cylindrical guideI6 and the high frequency path across the diameter of the guide i6 iscompleted from the cylinder 2! by means of a conductive tube 30 whichsurrounds the lead 29.

The arrangement of Fig. 6 operates substantially in the same manner asthat shown in Fig. 3 except that the electrons pass through the hole 23in disc 2| and are slowed down by the relatively low voltage of anode 24before striking the anode. The electromagnetic waves emerge from theresonant cavity through the space between the disc 2| and the right-handwall of cylinder and thence by way of the space between the tubes 26 and21 and through the insulating ring 28 into the interior of the waveguide Hi. The return circuit for the anode 24 is over the lead 29shielded by the tube 38.

Figs. 7, 8 and 9 illustrate an alternative construction of the resonatorwhich avoids the use of the insulating ring 3 and whatever dielectricloss may be associated with the means for maintaining the desiredseparation between the cylinders and 2. Although in the arrangementshereinabove described, the amount of dielectric employed is small andwhat there is may be placed at a nodal point, the arrangements shown 'inFigs. 7, 8 and 9 completely avoid this source of dielectric loss. Inaddition these arrangements enable a plurality of electron beams to beused with a single resonator of substantially the same dimensions asthose shown in the preceding figures. A further feature Of thearrangement is that provision may be made for a voltage stepup betweenthe first and second gaps traversed by the electron beam. Referring toFig. 7, the outer cylinder is shown at 3|. The inner cylinder 32 is heldin position coaxial within the cylinder 3| by studs 33 and 34.Arrangements are provided to accommodate a plurality of electron streamsin a cylindrical array about the axis of the resonator. The location ofa typical array of electron beams is indicated in Fig. 9 Where eightelectron gun assemblies are arranged in a circle as indicated at 35. Theradius of the circle is approximately a quarter of a wave-length for thewaves as propagated in the resonator. The electron guns and associatedvacuum tubes, batteries, etc., are essentially the same as for thesingle electron beam shown in the earlier figures. Each electron beam isprovided with aligned apertures and the right-hand wall of the cylinder3| serves as a common target for all the electron beams. The effectivelength of the resonating cavity from the stud 33 arormd peripherally ineither direction to the stud 34 is substantially one completewave-length at the operating frequency. The plurality of electron beamsmay include any number up to the limit that can be accommodated in thespace. The arrangement gives an approximation to the ideal condition ofa continuous ring ortubular electron beam. In practice, however, a smallnumber of beams from perhaps six upwards will usually suffice.

It will be noted that in the arrangements of Fig. 1 and other earlyfigures, the intensity of the electromagnetic field at the two gapstraversed by the electron stream is the same. In other Words in avelocity variation arrangement according to these figures, the magnitudeof the high frequency field which modulates the velocities of theelectrons is the same as the magnitude of the field at the point wherethe energy is extracted from the grouped electrons. This is fundamentalto the arrangements so far clescribed. Fig. 8 shows an arrangementwhereby this limitation may be avoided. The resonant cavity on theleft-hand side in Fig. 8, instead of extending substantially to theaxis, is terminated by a cylindrical wall 36. The wave-length of theresonant cavity extends from the wall 36 radially outward, then axiallybetween the cylindrical walls 3| and 32 and thence radially inward tothe stud 34. While the distance from the right hand gap radially inwardto the stud 34 is preferably a quarter wave-length, the distance fromthe left-hand gap to the cylindrical wall 36 is less than a quarterwave-length and may be designed in any desired proportion with respectto the quarter wave-length. Accordingly, the righthand gap is located ata voltage anti-node as in the case of Fig. 1, but the left-hand gapwhere the electron velocities are modulated has a substantially smallervoltage impressed across it. In

this manner a voltage step-up may be introduced which can be usedadvantageously to increase the efiiciency of the system.

Figs. 10 and 11 show an arrangement whereby a substantially circularresonant chamber, adjustable for tuning and having electrodes for theaccommodation of an electron stream may be inserted in a rectangularsection of wave guide. The walls of the guide are shown at 94. One endof the resonator is formed by a block stationary with respect to thewave guide and having a circular cylindrical portion cut out on theright-hand side. An aperture 96 in the block Q5 communicates between themain portion of the waveguide and the circular resonant chamber. Theright-hand side of the resonant chamber is closed by means of a slidablymounted block 9'! which has a circular cylindrical portion cut from theleft-hand side. The block 91 may be adexcept that the lower justed inposition by any suitable screw-threaded device. operated by means of ahandle 98. Conical apertured electrodes 59 and Ill-II are set into theupper andv lower walls of the Waveguide near the center of the circularcylindrical enclosure between the blocks 95 and 9-2. The gap between theelectrodes 99 and Ito may be a modulating gap or an output gap accordingto the desired application. The tuning feature of the resona-- tor isadvantageous when it is designed to operate a system at any selectedfrequency over a predetermined range of frequencies.

. Fig. 12 shows a resonant system corresponding generally to theresonator of Fig. 1 with provision for an electron beam to beintroduced, sections of rectangular wave guide being employed, however,in place of concentric cylindrical shells. The wall IE3! encloses the:entire structure and serves to hold the inner portions of the structurein place without the use of any insulating members.

The arrangement of Fig. 12 consists of .two rectangular guides I50, I51,55-2 and I53, I54, I55 of width somewhat greater than Air/2, where )\ais the free-space wave-length, and, of length equal to le/Z, where. tois the corresponding wavelength in the guide. These are folded in themanner shown so that their ends are juxtaposed at gaps I56 and I51. Arectangular box I58 of width somewhat less than Ra/Z joins the twoguides, and constitutes a drift tube. An electron stream in the form ofa sheet enters from an electron gun comprising a linearly extendedcathode I02. The electron sheet passes across gap I56, where it receivesa velocity variation, and then, after electron bunching has occurred inthe drift tube I58, it crosses the gap it? where it delivers energy tothe system. It will be noted that no dielectric material is required tosupport parts within the resonant chamber. Also, by virtue of the linearextension of the electron source to dimensions approximating a halfwave-length, the system is enabled to utilize very large values ofelectron current. In order to fix the nodal points of the oscillation atsuch points that the gaps I56 and I57 will be at loops of. the standing.wave in the resonant system, partial transverse barriers I04 and IE5 maym provided. In practice, the cathode I62 may have an active length ofapproximately one-half of the free-space wavelength before complicationsdue to the length of the filament begin to cause trouble. Couplingarrangements, electron collector and hermetic sealing are not shown butmay readily be supplied in any suitable manner.

Fig. 13 shows a somewhat similar arrangement side of the rectangulararrangement of wave guides is omitted and tuning systems I05 and IE1 areused to close off the otherwise open end of the wave guide. Arectangular guide IE8, WI, I62, I63, hi l of width somewhat greater thanlie/2 is provided with transverse slits I65, E86, 36? and I58 in thewalls, and folded over in such fashion that the slits are aligned. Thetwo folded portions are connected by a drift tube I59, and an electronsheet is projected through the slits. The pistons I95 and H31 areadjusted to give the guide an efiective length of approximately M}. Ingeneral, the distance from the gap IB'I, I68 to the piston IIlI will bemade equal to substantially [kc/4, so that at the gap I67, I68 there mayexist the maximum field intensity. For best efficiency the distance fromthe gap I65, I66 to the piston IIJB is usually less than Act/ l. Ifdesired, the guide section ISI, I62,

I63 may be made adjustableby means of trombone-type sliding joints sothatv when the distance from the gap I65, I66 to piston I06 has beenadjusted, a final tuning adjustment may be made by tuning the trombonesection ISI, I62, I63. The linearly extended cathode is accommodated asin. the arrangement of Fig. l2.v

Fig... 14- shows in more detail the relation of. the cathode I02 to the.guide and includes a oathode heating battery I03.

The use of the linear cathode to increase the power capacity of. anoscillator or amplifier may be extended to types of structures otherthan those. having, two gaps to be traversed by electrons and operatingupon the velocity variation principle. For example, a diode oscillatorof the type disclosed by F. B. Llewellyn in U. S. Patent 2,190,868,.issued February 20, 1940, may be built into a sectionof squareorrectangular wave guide as shown. in, Figs; 15 and 16, The filament orcathode. heater I2U extends across the entire width of the guide and isaccessible to the outside at its ends: which may protrude through insulating bushings. An electron emitter I2-I may be placed in proximityto the filament I23 and the emitter preferably extends substantiallyacross the width of the guide, but is insulated from the guide asbymeansof a support I22. Conductive partitions I23 and I24 for constricting theguide are conductively connected to the side walls and extend from sideto side. Insulatedfocussing electrodes I25 and I26 are set intothemember I23- and are separated by a gap. I21 transverse to thelongitudinal axis cf the guide. Another gap I28 extends transverselythrough the member'I'24 and below the gap I28 is fastened an insulatedelectron collector I29. The vacuum chamber may be closed by aninsulating partition I30; which may be of glass, and a conductive endwall I3I. At a distance to the left of the gap I2! and symmetrical withwall I3-I may be located a coupling iris 132, preferably outside of thevacuum chamber. The batteries I 3-3, 134,135 and I38, as shown in Fig.15A are employed respectively for filament heating, applying focussingvpotential to electrodes 'I25and I26, applying accelerating. potential tothe members I23 and i2 3, and applying a retarding potential to thecollector I29. The battery connections, shown in Fig. 15A, are omittedfrom Fig. 15' for greater clarity.

The operation of. the device of. Figs. 15. and 15A is similar to that ofthe. arrangement of Fig; 10 of the Llewellyn patent, above cited. Thespacing between wall I3I and iris. I32 is determinedso asto make the.wave guide section to th'e'right of the iris I32 resonant at a desiredoperating frequency. The. spacing in. the. restricted portion betweenmembers I23. and I24 is arranged in conjunction with the acceleratingpotential to provide an electron transit time between I23 and IZd equalto substantially /4, /4 or /4, etc), cycles at the operating frequency,as explained by Llewellyn. Electrons from the cathode I2! are focussedthrough the gap or slitl I21. into the gap between the. members i23 andI24 which gap, as mentioned, is of such length that a critical transittime relation for the production. of negative resistance in the electronstream is satisi'ied. After next passing through the. second slit I28,the electrons are collected by the electrode I29. When the system isproperly adjusted, high frequency energy from the resonant section maybe supplied to an external load through iris I32 and the portion of waveguide extending-to the left of the iris.

Fig. 16 shows an extension of the arrangement of Fig. 15, whereinseveral gaps are provided, to be traversed successively by the electronstream, each gap having the proper length to individually satisfy thecritical transit time relation. When properly adjusted each gap suppliesenergy to sustain the oscillations in the resonant section of guide. Thegaps are formed between the members I23 and E24 by the introduction ofapertured plates as indicated at 537 and $38.

Fig. 17 shows an oscillator in which a wave guide is bent aroundin'aU-shape so as to intercept the electron stream at two points deter"mined by thegaps-lfifl and I99. and an iris I #3 are adjusted topositions approxi mately one-quarter wave-length either side of the gapI09 as shown, and irises I H and H2 are placed at approximately quarterwave-length distances either side of the gap 138. With proper adjustmentthe interaction of the system with an electron stream in the gaps 08 and9 will result in standing waves being maintained in the wave guidebetween the piston H0 and the iris HI. It has been found that when theiris apertures are small and the losses in the walls of the wave guidesmall, a standing wave of very high amplitude is readily maintainedeither between Ill and H2 or between H0 and US. By adjusting theposition of the piston H0 the two resonators may be made to have thesame frequency. The bent section H4 serves to couple the two resonatorsand constitutes a feedback line or guide. A substantially pure travelingwave with practically no reflection or attenuation is set up in thesection I I4 and sustained oscillations are readily maintained in thesystem by interaction between the electron stream and theelectromagnetic fields in the gaps H38 and H39. 4 14 may be of anyconvenient length and is preferably adjustable as by means of atrombonetype slide, so that the relative phases of the oscillations inthe input and output stages can be given a suitable value.

What is claimed is:

1. A resonator comprising a hollow cylindrical shell of conductivematerial substantially closed by plane conductive end plates, and acoaxial I cylindrical conductive core of slightly shorter length andslightly smaller diameter than said shell, said core being positionedwithin said shell and spaced therefrom by means comprising coax ialcylindrical conductive spacers at either end of said core, said spacersbeing of materially different radius at the two ends of the core.

2. A toroidal hollow resonator of substantially U-shaped cross sectionhaving aligned apertures on a line parallel to the axis of saidresonator Wave to which said resonator is resonant.

3. A toroidal hollow resonator of substantially U-shaped cross sectionhaving aligned apertures on a pluralit of lines parallel to the axis ofthe resonator, said lines passing through one of the arms of the saidU-shaped cross section at substantially a quarter wave-length from oneend of the U-shaped resonant space.

4. In combination with a resonator in accordance with claim electronbeam-producing means for sending electron beams through a plurality ofsuccessions of aligned apertures.

5. A closed hollow resonator having a generally U-shaped sectionalconfiguration, and means The section A piston HS chamber and into theother of said portions.

7. A folded full wave-length resonant chamber having portions inproximity that are separated by from one-half to three-quarters of awavelength as determined'within the chamber by the standingelectromagnetic waves therein, and means to project a stream ofelectrons into and through a portion of said resonant chamber materiallyless than a quarter wave-length from one extreme limiting portion ofsaid resonant chamber and thereafter into a portion of said resonantchamber substantially at a quarter wave-length from the other extremelimiting portion of said resonant chamber.

8. A full wave-length resonant section of wave guide closed at the endsand having a configuration as if folded upon itself to bring intoproximity portions thereof that are separated at least a halfwave-length as measured along the length of the wave guide, and means toproject an electron stream through said resonant section at one of saidportions and into said resonant section again at the other of saidportions.

9. A contorted full wave-length cavity resonator having portions inproximity that are separated at least a half wave-length as measuredwithin the resonator, and means to project an electron stream throughsaid resonator at one of said portions and into said resonator again atthe other of said portions.

10. A cavity resonator of integral wave-length, closed at the ends andfolded upon itself to bring I into proximity portions of said resonatorseparated at least a half wave-length as measured within the resonator,and means to project an electron stream through said resonant section atone of said portions and into said resonant section again at the otherof said portions.

1 A length of wave guide substantially closed at the ends and foldedupon itself, and means to project an electron stream successivelythrough said wave guide at one point and into the said wave guide againat a point in the opposite fold.

12. A hollow outer cylindrical conductor, a pair of conductive endplates therefor, an inner cylindrical conductor within said hollowconductor, said inner cylindrical conductor having conductive endsurfaces, a pair of cylindrical conductive connectors of different radiiless than the radius of said inner cylindrical conductor connecting theend surfaces of said inner cylindrical conductor with the respectiv endplates of said outer cylindrical conductor at either end, whereby aresonant chamber is defined by said inner and outer cylindricalconductors, the end surfaces of said inner cylindrical conductor and theend plates of said outer cylindrical conductor together with the saidconnectors, and means to project a stream of electrons through the saidend plate adjacent the cylindrical connnector of larger radius into andthrough a portion of said esonant chamber, through the interior of said'11 inner cylindrical conductorand into another por- 't'ion of saidresonant chamber, said electron stream lying p'arallel'tothe common axisof said cylindrical conductors.

13. A hollow resonatorcomprisingthe" space between an outer hollowcylinder with plane ends, and an inner coaxial cylinder with plane ends,eX- cept as limited by a cylindrical spacer at either end, said spacersbeing'oi unequal radii.

14. A hollow outer cylindrical conductor, a pair of conductive endplates therefor, an inner cylindrical conductor within said hollowconductor, said inner' cylindrical conductor having conductive endsurfaces; a pair ofcylindricalconductive connectorsof unequal radius,each of said connectors being of radius. less than the said innercylindrical conductor, andsaid connectors connecting the end suriaces'ofsaid inner cylindrical conductor with the respective end plates of saidouter cylindrical conductor at either end, whereby a resonant chamber isdefined by said inner and outer cylindrical conductors, the end surfacesof said inner cylindrical conductor and the end plates of said outercylindrical conductor together with the said connectors, and meanstoproject a stream of electrons through one of said end plates into andthrough a portion of said resonant chamber, through the interior of saidinner cylindrical conductor and into another portion of said resonantchamber.

'ARNOIJD BOWEN.

